Recovery of krypton and xenon



M. P. HNlLlCKA RECOVERY OF KRYPTONAND XENON Original Filed April 28, 1950 I May 28, 1957 2 Shee ts-Sheet 1 KLEIN saaaaosov cunon mum MILOSLAV P. HNILICKA ATTORNEY y 28,1957 M. P. HNILICKA 2,793,507

RECOVERY OF KRYPTON AND'XENON Original Filed April 28, 1950 2 Sheets-Sheet 2 FIG. 2

GAS ADSORBERS LIQUID ABSORBERS ADSORBING 0 3 INVENTOR.

MILOSLAV P. HNILIICKA 43214 )n- A 7' TORNE United States Patent RECOVERY OF KRYPTON AND XENON Miloslav P. Hnilicka, Concord, Mass., assignor to Amoco Chemicals Corporation, a corporation of Delaware Original application April 28, 1950, Serial No. 158,639,

now Patent No. 2,698,523, dated January 4, 1955. Divided and this application December 17, 1954, Serial No. 475,099

5 Claims. (Cl. 62---122) This invention relates to the recovery of krypton and xenon from air, and particularly, is directed to a process and apparatus for obtaining krypton and xenon from air as a by-product during the recovery of oxygen.

During the development of the electric light bulb, it was found that the candle power could be increased and the life of the tungsten filament prolonged by the use of inert gases with high atomic weights. Specifically, it was discovered that nitrogen, and later argon, were particularly valuable. Further research and development in the electric bulb industry has proved that the rare gas krypton having an atomic weight of 83.70 would bring even greater improvement. However, there have been no successful means of producing krypton in sufficient quantity or economically for wide-scale use, for krypton is present in air in the extreme dilution of 0.9 part per million by volume.

The potential market for krypton, if used for incandescent bulbs at the present rate of manufacture, can be very conservatively estimated at between 1,000 and 2,000 S. C. F. D. of krypton.

Another market for krypton is the fluorescent lamp; however, the present small production and high price of this rare gas makes it unavailable.

Xenon is a rare and inert gas with a higher boiling point than krypton of 160.8 F. at atmospheric pressure. It is present in atmospheric air in extreme dilution of 0.09 part per million by volume. It is distinguished by very favorable discharge light spectrum similar to the daylight. At the present time, special electric discharge lamps with xenon are used as an intensive flash light source in photography and high intensity lighting purposes of runways at airports. The brightness of these flash lamps exceeds many times that of the sun. At thepresent time, available supply of xenon is even more restricted and market prices higher than those of krypton, because production of xenon requires processing and liquefaction of extremely large volumes of air.

The recovery of krypton and xenon from atmospheric gases, or liquefied gases by low temperature adsorption has been known, but no effective means of recovery of the desorbed gases has been evolved. Adsorption materials such as silica gel or charcoal have been used in the adsorption of krypton and this method has been used for laboratory practice and small scale operations. However, it must be borne in mind that the major difliculty of industrial recovery of krypton and 2 an impurity in krypton and xenon and as an explosion hazard during the recovery process.

It is an important object of this invention to provide a method and apparatus for effectively and economically recovering krypton from air and removing the hydrocarbon impurities Which are adsorbed with the krypton and xenon during the process.

A further object of the invention is to provide a method and apparatus for recovering krypton and xenon which may be used in conjunction with an oxygen recovery system and which effectively disposes of the hydrocarbon impurities and the explosion hazard incident to their presence in such a system. A still further object of the invention is to provide a practical means: of recovering krypton and xenon in relatively pure form.

The process and apparatus of this invention will be described with reference to the drawings, wherein:

Figure 1 is a diagrammatic arrangement of the krypton and xenon recovery apparatus of this invention, illustrating the first phase of the recovery;

Figure 2 is a diagrammatic arrangement of the portion of the apparatus shown in Figure 1, taken along the line 22, indicating the second phase of the recover Y;

Figure 3 is a diagrammatic arrangement of a portion of the krypton and xenon recovery apparatus according to this invention illustrating another location of adsorbers during the first phase of recovery by adsorption in a conventional oxygen plant.

Figures 1 and 2, which schematically represent the salient features of a process for producing oxygen from air, include a heat exchanger 10, a high pressure air fractionator 11 and a low pressure oxygen fractionator 12. To this oxygen recovery apparatus is added the krypton and xenon recovery apparatus which includes a pair of gas adsorbers 13 and 14 and/ or a pair of liquid adsorbers 15 and 16. Each pair of adsorbers is so ar ranged with switch valves that they may be used alternatively with one adsorber, on-stream, actively adsorbing, and the other off-stream and desorbing. The active adsorber 13 of the first pair is preferably maintained at 272 F. and the active adsorber 15 of the second pair is preferably maintained at 252" F. Also included in the apparatus are two condensing coils 17 and 18.

During the first phase of the krypton and xenon recovery, as shown in Figure l, the condensing coil 17, for condensing acetylene, is preferably operated at 150 F. and the condensing coil 18 for condensing krypton and xenon is preferably operated at 289 F. An auxiliary refrigeration system 19 employing a suitable refrigerant as, for example, ethylene is used to maintain the condensing coil 17 at -l50 F. and liquid oxygen 56 from the bottom of the low pressure oxygen fractionatoi 12 is utilized for maintaining the condenser 18 at approximately --290 F.

The first phase of krypton and xenon recovery process comprises introducing a stream of compressed air at pounds pressure into the system through feed line 20. The atmospheric air contains the usual constituents such as nitrogen, oxygen, rare gases, including krypton and xenon, and also contains certain hydrocarbon impurities including acetylene. The air passes into the heat exchanger 10, which is typical of any recuperative or regenerative exchanger system, wherein it is cooled by the cold nitrogen and oxygen passing, respectively through lines 21 and 22, the nitrogen and oxygen having a temperature of approximately -300 F. at the time they enter the heat exchanger.

After being cooled, the air stream, from which moisture and carbon dioxide have been largely removed, issues from the heat exchanger 10 through line 23, flows into the gas adsorbing chamber 13, which as illustrated in Figure l is the gas adsorbing chamber which is oil-stream. The air flows through a bed of silica gel or other suitable adsorbing material contained in the adsorbing chamber 13. The temperature of the air as it flows through the bed is about 272 F. and krypton, xenon, and acetylene which are in the cold air stream are adsorbed over a period of time, until the silica gel is largely saturated, whereupon the switch valves 24 and 25' are turned to divert the stream through the alternate gas adsorber 14.

After the cold air has passed from the adsorber 13 and is largely free from the acetylene, krypton, and xenon previously contained therein, it continues in the usual way, part of it constituting a high pressure air feed passing through line 26 to the high pressure air fractionator 11 and another part passing through line 27 to expander 28 and entering the low pressure oxygen fractionator 12 at 29.

111 the high pressure air fractionator 11 the liquid oxygen bottoms 55 are drawn off through line 3t and pass into liquid adsorber 15, or alternately 16, which are thus maintained at 275 F. and any acetylene, xenon, and krypton which are present in the liquid stream are therein adsorbed and the purified product is passed into the low pressure oxygen fractionator 12 through line 31.

An alternative arrangement for adsorption of krypton and xenon in a conventional oxygen plant is represented in Figure 3, wherein the adsorbers 60 and 61 are located in the exhaust line from expander 98 thereby taking advantage of better adsorption at lower temperatures. The exhaust from the expander at a temperature approximately 3{)O F. improves the ratio of partial pressure of krypton and xenon to corresponding saturation pressures of said gases to those of common air components which is theoretically desirable for better absorption.

Liquid adsorbers 62 and 63 are located in the oxygen drain line from main reboilers 113 to product evaporators. In such an arrangement, krypton and xenon are considerably concentrated because in reboiler 113 a major palt of the liquid oxygen is evaporated as reboil, necessary for the low pressure tower fractionation.

Krypton and xenon dissolved in oxygen and having relatively high boiling points remained in the bottoms of reboiler 113. The adsorption of these gases at a higher concentration from liquid oxygen is more effective and the size of equipment may be considerably smaller.

For a more complete description of the alternative arrangement of the adsorbers, reference is made to Figure 3. The air stream at 75 pounds pressure is introduced through line 90. The moisture and carbon dioxide have been previously removed by cooling to liquefaction temperature by heat exchange with oxygen products and nitrogen vents, carried respectively in lines 92 and 91. The air stream issues from heat exchanger 80 through line 93 part going directly to high pressure tower 81, through line 96, and part going through line 97 to expander turbine 98. By the expansion in turbine 98 from 75 pounds to atmospheric pressure, the air is cooled to approximately -300 F. and flows through reversing valve 94 to the gas adsorbing chamber 61 which, as illustrated in Figure 3, is the adsorber in the stream. In this arrangement, the temperature of the air as it enters the adsorber 61 and flows through the bed of silica gel or other suitable adsorbent material at nearly atmospheric pressure is 300 F. Krypton, xenon and acetylene contained in this air stream are cooled far below their boiling points. Conesquently, silica gel or other convenient adsorbent cooled to 300 F. will adsorb from the air stream, krypton, xenon and acetylene over a period of time until the silica gel is largely saturated, whereupon the switch valves 94 and 95 are turned to divert the stream of air through standby gas adsorber 60. The efiluent from the adsorber is largely free of rare gases and acetylene and continues its flow in the usual way through the line 99 to enter low pressure oxygen fractionator 82.

Krypton, xenon, and acetylene contained in the other 4 part of the process air are liquefied in the high pressure tower 81 and are dissolved in liquid oxygen in the bottom of low pressure tower 82. In reboilers 113, the major part of the liquid is evaporated to furnish reboil for final fractionation in tower 82. Krypton, xenon and acetylene are thus concentrated because of considerably higher boiling points and are drawn off from the reboilers through line 64. These oxygen bottoms enriched in krypton, xenon and acetylene enter the liquid adsorber 62 or alternately 63 at a temperature of approximately 288 F. Acetylene, krypton and xenon present in this stream in a higher concentration than originally in the liquefied atmospheric air in the line 100 from the high pressure tower 31 are therein adsorbed by silica gel or other adsorbent. Purified liquid oxygen, from which hazardous acetylene was removed, enters through line 67, the product of evaporator 59, in which it is completely evaporated. Gaseous oxygen from product evaporator 59 issues through line 71 to the line 92 and enters heat exchanger 80 where it is warmed up by heat exchange with incoming process air to atmospheric temperature. Liquid adsorbcrs 62 and 63 are provided with reversing valves 65 and 66 enabling switching of the liquid air stream when adsorbing capacity of the adsorber onstream is exhausted to a standby adsorber and vice versa.

It will be appreciated that the arrangement shown in Figure 3 may be substituted for the adsorbing circuit in the recovery system shown in Figure 1, and thus combined, may be utilized as an integral part of a klypton and xenon recovery system.

The relative quantity of adsorbed gases in an adsorbent as a given temperature depends upon the molecular structure and the ratio of partial pressures and corresponding saturation pressure of the vapors of the concerned gases at the same temperature. Because the boiling points of argon, 302 F.; oxygen, 297.4 E; nitrogen, 320 F.; are considerably lower than those of krypton, -252 F, and xenon, -160.6 E, the saturated vapor pressures at the adsorption temperature of argon, oxygen and nitrogen will be many times higher than those of krypton and xenon. Due to the marked diiterence of atomic weight and structure of the mentioned gases, krypton and xenon will have the highest adsorption affinity.

Although at the beginning of the cycle, all components of the air will be partially adsorbed, with advancing of the cycle, heavier and higher boiling point components will successively substitute the lighter components until the adsorbent capacity is exhausted. Therefore, in the temperature ranges indicated herein, at which the adsorbers are operated in the system of this invention, the process of adsorption is suitably selective in adsorbing krypton, xenon and acetylene.

Referring again to Figure 1, after the exhaustion of the adsorbing capacity of the adsorbers, krypton, xenon and other gases, including hydrocarbon, are desorbed from the silica gel by means of evacuation with a vacuum pump and a controlled increase of temperature.

The gases from the gas adsorbers 13 and 14 and from the liquid adsorbers 15 and 16 are desorbed by controlled increase of temperature by internal heating coils in the adsorbers or by non-condensable gases passed therethrough from the recycle heater 34. The desorbed gases are brought through the switch valve 24 from adsorber 14 and valve 9 from adsorber 16 to vacuum pump 72. The vacuum pump creates a partial vacuum on the adsorber which facilitates the desorption of the adsorbent and forces the desorbed gases through cooler 73 at suitable condensation pressure. The gases are cooled down in cooler 73 and passes through valve to the first condensing coil 17 and are cooled to approximately F. by refrigeration unit 19, consisting of compressor 57 and condenser 58, using a suitable low temperature refrigerant such as, for example, ethylene. Acetylene precipitates in solid form on the walls of condensing coil 17. Krypton and xenon and the non-condensable carrier gases, all having grease? boiling points below the temperature of condenser 44, issue through line 38 and open valve 42 to second condenser 45. Krypton and xenon precipitate in solid form in coil 18 cooled to -289 F. by liquid oxygen 56 conveniently supplied through line 170 from the bottom of low pressure fractionating tower 12. Oxygen vapors evaporated in the condenser 45 by cooling of the gases and by the condensation of krypton and xenon in coil 18 are returned as part of the cold oxygen product and may be routed through line 162 to heat exchange unit 10.

. The non-condensable carrier gases leaving coil 18 contain a small quantity of krypton and xenon vapors corresponding to vapor pressures of these gases at condensation temperature. In order to increase recovery yield, the system of non-condensable carrier gases is recycled. The pressure is reduced by a throttle valve 41 and recycle gas is sent through line 39 to heater 34. Warm recycle gases from heater 34 reenter by line 35, gas adsorber 14 and by line 36 to liquid adsorber 16. The stream of noncondensable gases is again enriched in krypton, xenon and acetylene and is removed by the vacuum pump 72 through lines 8 and 37.

The pressure in the condenser system may be controlled by venting surplus non-condensable gases leaving the second condenser 45 by means of valve 125. Desorption pressure can be adjusted by the pressure reducing or throttle valve 41 for best selectivity of desorption.

The above described recirculation may be continued until all the krypton, xenon and acetylene gases are desorbed from the adsorbers and the adsorbent is completely regenerated.

A controlled desorption may be used to provide an opportunity to obtain selective desorption at different temperature levels, i. e., fractions containing successively, major parts of krypton, xenon and little of hydrocarbons, and thereafter major parts of hydrocarbons, including acetylene, and little krypton and xenon, to thus facilitate further separation of krypton and xenon from the hydrocarbon impurities.

Total heat losses of the desorption and the subsequent precipitation of acetylene, krypton and xenon in condensing coils can be reduced greatly in industrial installations by using countercurrent heat exchangers. However, the heat exchangers form no essential part of the system for the recovery of the rare gases, and, therefore, have not been included in the drawings.

In the second phase of the recovery of krypton, and xenon illustrated schematically in Figure 2, the condensing coils 17 and 18 are shut otf periodically from the purge stream carrying acetylene, krypton, and xenon from the adsorbers, by closing valve 40 in conduit 37. Also valves 41 and 42 are closed to isolate the condensing coils 17 and 13 from each other and to close the connections of coil 18 with the desorbing cycle of the apparatus. The coils 17 and 18 are thereupon suitably warmed; one effective means being to drain the ethylene and liquid oxygen from the coil containing chambers 44 and 45 and introducing a Warm stream of gas or liquid into the chambers through feed lines 46 and 47, as shown in Figure 2.

In the final clearing of the condensers 17 and 18, which have been sufficiently warmed to re-evaporate or sublime the condensed substances contained therein, application of vacuum or a stream of a suitable carrier gas as nitrogen, CO2 or steam may be employed, and, in fact, one type of gas may be used for clearing one condenser and another carrier gas for clearing the other. For example, steam or CO2 may be used for carrying out the acetylene and other hydrocarbons in the higher temperature condenser 17 and nitrogen may be used to carry out the krypton and xenon contained in condenser 18.

In the apparatus as shown, it is contemplated that, as the coils are warmed, a stream of nitrogen will be introduced into both coils passing through feed line 32,

part of the stream being diverted through line 48 to coil 17 and part of the stream through line 49 to coil 18. During this phase of the operation, valves 50 and 51 are opened to admit the stream of nitrogen into the respective coils. The hydrocarbons previously condensed in the coils are evaporated and are carried off by the carrier gas through conduit 52. During this phase of the opera-,

tion, valve 53 is opened to permit the passage of the acetylene-carrying gas. The mixture in the carrier gas issuing through conduit 52 contains acetylene and other hydrocarbons and also includes a small amount of krypton and xenon.

As the nitrogen stream passes into the coil 18 through line 49 the kryptonand xenon which was previously condensed in the coil 18 is evaporated or sublimed and removed from the coils by the carrier gas through conduit 54-, valve 55 being opened to permit the passage of the krypton and xenon bearing gas through line 54. This stream, in addition to the krypton and xenon, also contains a small amount of acetylene.

The krypton and xenon, sublimed from the condensing coils, will be present in the carrier gas in a much higher concentration than had existed previously in the air or in the oxygen bottoms from the low pressure fractionator used in conventional methods for recovery of krypton and it will also be in higher concentration than it was in the desorbing nitrogen purge stream.

The final fractionation of xenon and krypton from this carrier stream, their separation and purification, is considerably easier to accomplish than from mixtures produced by present methods. From the krypton and xenon bearing stream the first step of purifying is to remove any trace of hydrocarbons which were not separated in the condensing apparatus, the removal may be effected by catalytic burning in copper oxide furnaces and the carbon dioxide and water formed by the burning of the hydrocarbons in these furnaces can thereafter be removed by a caustic wash or by other convenient means.

It is evident that the acetylene explosion hazards which previously have been the cause of considerable difiiculty in recovering krypton and xenon are practically eliminated by the fact that with the method of this invention,

the acetylene, xenon and krypton mixture is handled in a nitrogen stream or in an inert carrier gas, instead of in an oxygen stream, as has been used in previous methods.

The final separation and purification of krypton and xenon from the nitrogen carrier stream may be effected by the conventional fractionation at low temperature and increased pressure. This method is well known and is particularly efficient because there is a larger temperature difierential between the respective boiling points of the xenon-krypton-nitrogen combination than between the respective boiling points of the combination of kryptonoxygen.

It will be appreciated that this invention, in addition to providing an efiicient and eflective method and apparatus for recovering xenon and grypton and for eliminating the explosion hazard during the recovery, also reduces the explosion hazard in the oxygen system by removing the acetylene impurities from the air before it is introduced into the high pressure air fractionator 11 and the low pressure oxygen fractionator 12. In the past, the hazard of explosion was present because of the dangerous accumulations of hydrocarbon impurities in the liquid bottoms of the fractionators during the recovery of oxygen.

During the operation of the xenon krypton recovery apparatus of this invention the main streams passing through the active adsorbers are never interrupted and they are continually being subjected to adsorption. This is possible because the desorption cycle is considerably shorter than the adsorption cycle and therefore the inactive adsorbers may be completely desorbed and then temporarily shut off from the condensers while the latter are cleared during the adsorbing cycle of the active adsorber.

It will be appreciated that the advantages of the apparatus and method of this invention are found in the fact that the refrigeration loss and power requirements are considerably less than those required by conventional methods because extraction of krypton and xenon by adsorption from the air stream does not require any additional refrigeration over a comparatively long period as do other methods which involve the continuous separation of krypton from oxygen bottoms by fractionation of large quantities of liquid oxygen with a low concentration of krypton, or xenon.

Furthermore, the equipment required for krypton and xenon recovery by low temperature adsorption is relatively simple and inexpensive. The two pairs of reversing adsorbers are simple and may be small vessels and the desorption and krypton and xenon concentration equipment, operated intermittently, is handling small quantities of purge and carrier gases and reduces very considerably the required investment.

It will be appreciated that the temperatures and pressures indicated at which the various members of the ape paratus are operated are only illustrative and the apparatus may be effectively operated at other temperatures and pressures, providing the temperatures and pressures are in the range to eifect the several physical changes in the materials which are being treated; particularly the air which initially is introduced into the absorbing chambers should be sufiiciently cooled, preferably below the boiling points of krypton and xenon and of the hydrocarbon impurities but above the boiling points of oxygen and nitrogen in order to increase relative saturation of vapor of said rare gases and thus effect the adsorption of the krypton and the hydrocarbons.

The first condensing coils should be maintained below the melting point of the hydrocarbon impurities and particularly the acetylene, but above the melting point of krypton and xenon, and the second condensing coil should be maintained below the melting point of krypton and xenon to effect the selective condensation of the hydrocarbon impurities in the first condensing coil and the condensation of the krypton in the second condensing coil.

This is a division of my co-pending application, U. S. Serial Number 158,639, filed April 28, 1950, now Patent No. 2,698,523.

I claim:

1. Apparatus for producing krypton and xenon from the cold, compressed air stream of an oxygen process comprising in combination a pair of selective adsorbers and switch valves for alternately introducing said air stream into the adsorbers whereby krypton, xenon and hydrocarbon impurities are continuously being adsorbed from said stream, and means for heating the inactive adsorbent under reduced pressure whereby the hydrocarbon impurities, xenon and krypton are selectively desorbed from said adsorbent while the latter is out of contact with the cold air stream.

2. Apparatus for producing krypton and xenon from the cold, compressed air stream of an oxygen process comprising in combination a pair of selective adsorbers, switch valves for alternately introducing the air stream into one of the adsorbers whereby krypton and xenon are continuously being adsorbed from the air stream, means for warming the other inactive adsorber and vacuum means for evacuating the krypton and xenon from said inactive adsorber.

3. Apparatus for producing krypton and xenon from the cold, compressed air stream of an oxygen process comprising in combination at least one pair of selective adsorbers and switch valves for alternately introducing said air stream into at least one of the adsorbers whereby krypton, xenon and hydrocarbon impurities are continuously being adsorbed from said stream, and means for simultaneously heating at least one of the adsorbers under reduced pressure first to a temperature sufi'icient to drive off the krypton and xenon and then to a higher temperature to drive off the hydrocarbon impurities 4. A method for recovering krypton and xenon from air and separating them from the hydrocarbon impurities in the air comprising passing a stream of air cooled below the boiling point of krypton into contact with an adsorbent material to thereby adsorb the krypton and xenon and the hydrocarbon impurities in the air stream, desorbing xenon and krypton by raising the temperature of said adsorbent material and evacuating the desorbed gases from the adsorbent material, compressing the desorbed gases to condensation pressure and cooling the compressed gases, thereafter selectively condensing and freezing out the hydrocarbon impurities and the krypton and xenon, and thereafter recovering krypton and xenon substantially free from said impurities.

5. In a process for producing oxygen from air in which a continuous stream of cold air under pressure flows to fractionating equipment for separation into its constituents, the method of removing the rare gases and hydrocarbon impurities from said cold air stream without interrupting the continuous flow of said stream, which comprises alternately introducing said air stream into adsorbers at least one of which is continuously maintained in said stream and is active for continuously adsorbing krypton, xenon and said impurities from said stream and at least another of which is inactive and is being desorbed and revivified during its period of inactivity by warming and evacuation of the krypton, xenon, and said impurities previously adsorbed therein compressing the resulting desorbed gases to condensation pressure and cooling the compressed gases, thereafter selectively condensing and freezing out said impurities and the krypton and xenon, and warming and reducing the pressure of any residual noncondensable gases and recycling them into contact with the adsorbent material, said noncondensable gases being withdrawn with desorbed gases from the said adsorbent material when the desorbed gases are next evacuated.

No references cited. 

1. A PPARATUS FOR PRODUCING KRYPTON AND XENON FROM THE COLD, COMPRESSED AIR STREAM OFF AN OXYGEN PROCESS COMPRISING IN COMBINATION A PAIR OF SELECTIVE ADSORBERS AND SWITCH VALVES FOR ALTERNATELY INTRODUCING SAID AIR STREAM INTO THE ADSORBERS WHEREBY KRYPTON, XENON AND HYDROCARBON IMPURITIES ARE CONTINUOUSLY BEING ADSROBED FROM SAID STREAM, AND MEANS FOR HEATING THE INACTIVE ADSORBENT UNDER REDUCED PRESSURE WHEREBY THE HYDROCARBON IMPURITIES, XENON AND KRYPTON ARE SELECTIVELY DESORBED FROM SAID ADSORBENT WHILE THE LATTER IS OUT OF CONTACT WITH THE COLD AIR STREAM. 